ATMOSPHERIC STABILITY CLASSIFICATION

  Over the last several decades, a number of classification systems have been proposed to define atmospheric stability. Some of these schemes are explained below.

P- G Method

P- G method is one of the oldest methods to define atmospheric stability. The method requires the measurements of

The approach is based on original field work. The cloud observations are expensive. The scheme is given in the following table:

PASQUILL- GIFFORD STABILITY CATEGORIES

Surface Wind
( Measured at 10 m )
( m / sec. )
Surface Wind
( Measured at 10 m )
( mph )
Day 
Incoming Solar Radiation**
( Insolation )
( Strong )
Day 
Incoming Solar Radiation**
( Insolation )
( Moderate )
Day 
Incoming Solar Radiation**
( Insolation )
( Slight )
Night*
( Thin Overlast 
or
>= 4/8 cloudiness* )
Night*
( <= 3/8 cloudiness* )
< 2
< 4.5
A
A-B
B
F
F
2 - 3
4.5 - 6.7
A-B
B
C
E
F
3 - 5
6.7 - 11.2
B
B-C
C
D
E
5 - 6
11.2 - 13.4
C
C-D
D
D
D
6
13.4
C
D
D
D
D
*   Night is defined as the period from 1 hour before sunset to 1 hr. after sunrise
** Appropriate insolation categories may be determined through the use of sky cover and solar elevation information as follows :
Sky Cover 
( Opaque or Total )
Solar Elevation 
Angle > 60 o
Solar Elevation
Angle < 60 o
but, > 35 o
Solar Elevation
Angle < 35 o
but, > 15 o
4/8 or less or,
Any amount of High Thin Clouds
Strong
Moderate
Slight
5/8 to 7/8 Middle Clouds
( 700 ft. - 16000 ft. base )
Moderate
Slight
Slight
5/8 to 7/8 Low Clouds
( Less than 7000 foot base )
Slight
Slight
Slight

P-G / NWS Method

This is a variation of P-G method. The only difference between P-G method and this method is that on-site cloud cover observations are replaced by the observation taken at the National Weather Service (NWS) station in that area. The disadvantage of the method is the use of second-hand information for cloud cover. 
The STAR Method

The instrumentation for monitoring atmospheric stability is relatively expensive and is typically difficult to maintain. The instruments are used in remote locations where trained environmental scientists are not present, and at facilities where personnel are engaged in activities other than the hour by hour monitoring of the weather and/or at plants where it is required by law. The national Climatic Center ( NCC ) in Asheville, North Carolina utilizes a stability computation method which does not require special instruments but, rather, relies solely on the hourly weather observations made by meteorologists at National Weather Service ( NWS ) stations. Unstable conditions occur primarily during periods of strong surface heating and low wind speeds, and stable conditions occur only when the earth's heat is escaping to space ( high negative net radiation ) and winds are light. Also, neutral conditions can occur when it is cloudy and/or windy. The NCC method, then utilizes observations of cloud cover, made hourly by stations weather observer, coupled with the wind speed observed when the sky observation was made.

The data produced from the cloud cover and wind speed observations are referred to as STAR ( for STability ARray ) data. Historical STAR data are extremely useful because they can be obtained quickly from the NCC and they are inexpensive.


BNL Scheme

Brookhaven National Laboratory (BNL) scheme depends entirely on horizontal wind direction fluctuations. This system will not work if a definite relationship between horizontal fluctuation and vertical fluctuation does not exist at a site.

The classification are as follows:

Type A   : Fluctuations ( peak to peak ) of the horizontal wind direction exceeding 90o ( Extremely Unstable )

Type B2 : Fluctuations ranging from 40o - 90o

Type B1 : Fluctuations similar to A and B2, but confined to 15o and 45o limits ( Unstable )

Type C   : Fluctuations greater than 15o distinguished by the unbroken solid core of the trace ( Neutral )

Type D   : The trace approximates a line; short - term fluctuations do not exceed 15o ( Stable )

Note : Fluctuations are recorded over a 1 hr. period.

Sigma Phi Method:

The atmospheric stability can be determined using the standard deviation of elevation angle (phi) of the vertical wind direction. Sigma phi is a good indicator of the scale and intensity of the vertical motions of the atmosphere. The disadvantage of the method is that it requires a highly dedicated maintenance program to assure proper calibration of the system.

Sigma Omega Method:

In this method, sigma phi is indirectly calculated using the standard deviation of the vertical wind speed (sigma omega) and the average horizontal wind speed. This approach reduces the maintenance requirements of the sigma phi method.

Wind Direction Standard Deviation (Sigma Theta) Method:

The standard deviation of horizontal wind direction is an atmospheric stability classification system recommended by the Nuclear Regulatory Commission. The limits of sigma theta as given in Regulatory Guide 1.23 are given in the following table.

Modified Sigma Theta Method:

Mitchell and Timbre (1979) attempted to modify the NRC sigma theta method (MST) in order to account for lack of insolation during nighttime. The definition for nighttime was: one hour before sunset to one hour after sunrise. According to MST scheme, if stability class based on sigma theta at night is neutral (D) or stable (E,F,G), the stability class is based directly on sigma theta. If it is unstable (A,B,C), the observed wind speed and corresponding stability class are located in the following table and the associated stability class applicable to sigma z is identified. The MST method involves correction factors for plume meander that occur under low wind speed and nighttime conditions.

NRC Temperature Difference Method:

NRC Regulatory Guide 1.23 indicates the use of temperature differences with height for computing standard deviations of the plume. This method relates a set of ranges of temperature lapse rates (oC/100m) to the Pasquill-Gifford classes.

Wind Speed ratio (UR) Method:

Sedefian and Bennett (1980) suggested the use of wind speed ratio (UR) for defining the atmospheric stability. The relationship between stability and UR is shown below.

 

Four different atmospheric stability indicators have been examined by Scott-Waslikand & Kumar (1982) for their potential use in a radio nuclide dispersion model at a nuclear power plant.

The schemes are:

The other four schemes were not studied because sufficient meteorological data were unavailable. Inconsistencies were observed between the four schemes for the seven stability classes. The results of this analysis indicate:

The temperature difference and sigma theta methods do not correlate well in the determination of atmospheric stability. However, the two values were within one stability class of each other more than 80% of the time.

In its present form, the wind speed ratio method cannot be used for determination of atmospheric stability at the two coastal nuclear power plants that were examined.

Modified sigma theta (MST) is a small improvement over the conventional sigma theta method. However, MST is still not an acceptable substitute for the temperature difference method in the determination of atmospheric stability.